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Catalysts 2017, 7, 367 2 of 31 low. Large molecules are difficult to diffuse in micropores. They finally turn into carbon deposition, blocking the pore channel. The diffusion pathway length in conventional ZSM-5 zeolite is too long, which hinders the diffusion of molecule resulting in a low catalytic activity. Therefore, many studies have been conducted to overcome these limitations, such as preparation of zeolite with special morphology [11,12], hierarchical zeolite [13,14] or nano-sized zeolite [15], and element or compound modification [16]. Ryoo et al. [12] firstly prepared the nanosheet MFI zeolite in 2009. This nanosheet zeolite contained both microporous and mesoporous structure, which enhanced the diffusion of reactants. Besides, the nanosheets were very thin, which reduced the diffusion lengths and improved the catalytic activity further. It showed higher catalytic conversion of bulky molecules than conventional ZSM-5 zeolite. In addition, the hierarchical ZSM-5 zeolite was synthesized by adding soluble starch, sodium carboxymethyl cellulose [17] or anion exchange resin [18] as a second mesoporous template, and their catalytic activities were promoted with the increasing of mesopores by improving the diffusion efficiency and accessibility of acid sites. Overall, it can be summarized that the catalytic performance of ZSM-5 zeolite can be influenced by its pore channel structure, diffusion pathway length, acid amount and acid strength. Herein, we have reviewed the pioneered studies and recent developments about how to enhance the catalytic performance of ZSM-5 zeolite in catalytic cracking of hydrocarbons from the following aspects: (1) Synthesis of ZSM-5 zeolite with special morphology to decrease the diffusion pathway or increase the diffusion efficiency; (2) Synthesis of hierarchical ZSM-5 zeolite with extra mesopores or macropores to increase the diffusion efficiency of molecules and the accessibility of acid sites; (3) Synthesis of nano-sized ZSM-5 zeolite with short diffusion length to promote the diffusion of molecules and; (4) Optimization of the acid properties of ZSM-5 zeolite to enhance the catalytic activity. For each of these strategies, its advantages and limitations are discussed and compared with each other from the aspects of synthesis process, catalytic activity and potential for industrial application. 2. ZSM-5 Zeolite with Special Morphology ZSM-5 zeolites with special morphology are designed in order to decrease the diffusion length or enhance the diffusion efficiency, such as hollow ZSM-5 zeolite, nanosheet MFI zeolite, b-oriented MFI zeolite, nanorod-oriented zeolite, composite zeolite and fiber-sized zeolite and so on. When the diffusion length decreases or the diffusion efficiency increases, reactant and product could transform quickly to and away from the acid sites within the zeolite crystals and, thus, shift the selectivity to favor the production of more valuable products at reduced yields of coke [19]. 2.1. Hollow ZSM-5 Zeolite In hollow ZSM-5 zeolite with thin shell, the diffusion pathway can be reduced effectively compared with the conventional bulk ZSM-5 zeolite duo to its excellent textural properties (Table 1) resulting from the hollow voids in zeolite crystals. It has been proven that hollow ZSM-5 zeolite showed improved catalytic activity in hydrocarbon cracking by enhancing the diffusion of molecules [20]. It can be prepared by hard/soft-template method or selectively dissolving the inner part of zeolite crystals.PDF Image | Strategies to Enhance the Catalytic Performance of ZSM-5
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